۱- Introduction

۲- Materials and methods

۳- Results and discussion

۴- Conclusion

References

Abstract

The aim of this research was to investigate the methyl orange degradation using hydrodynamic cavitation. The synthetic solutions simulated a real textile effluent containing dye and metals (iron and nickel) as a consequence of the corrosion of nickel-plated metal components of the plant during the production of textile materials. In the first series of experiments, the hydrodynamic cavitation was studied in terms of operating inlet pressure and pH of solution. Subsequently, a full factorial plan was performed to determine the main effects and interactions among the investigated factors: inlet pressure (pin), temperature (T), initial dye concentration (cMO/0), and treatment time (t). The results showed that inlet pressure, temperature and concentration had a significant positive effect on dye degradation, as well as the interactions temperature/dye concentration and pressure/time. The optimum operating conditions among those investigated were: cMO/0 = 5 ppm, pH = 2, pin = 0.6 MPa and t = 1 h, independently of temperature (T). In the above conditions, the degradation yields were near to 75% and the final concentration was less than 1 ppm. On the contrary, at cMO/0 = 20 ppm, pH = 2, pin = 0.6 MPa, T = 40 °C and t = 1 h, the degradation efficiency was about 56% and the final concentration was less than 9 ppm. A comparison among the experiments carried out in the absence and in the presence of iron and nickel showed that metals acted as catalysts and the energy required for the process with metal ions was 5 times smaller than that those required by the experiments conducted without the metals in solution, at the same operating conditions.

Introduction

Textile industry produces wastewaters containing several types of dyes. In the dyeing process, around 15% of the colorants is lost and discharged as industrial residual solutions [1, 2]. The treatment of such liquid wastes is a matter of great concern, due to their colour and toxicity. It is estimated that the annual world production of synthetic dyes in 2009 exceeded 900,000 tonnes and it is expected to grow a million tonnes every year [3-5]. The azo dyes are the most commonly used dyestuff and represent around 50-70% of the total production [6-8]. This type of dyes is characterized by the presence of a double nitrogen bonds (-N-N-) and it is widely used in the textile, paper and cosmetic industries [2, 9]. The release into the environment of azo-contaminated wastewaters poses severe problems of toxicity because of their bio recalcitrance for conventional aerobic wastewater treatment and the toxic aromatic intermediates. Therefore, the developments of effective methods for the removal of such polluting dyes is of major importance. From a literature survey, a lot of papers deals with the traditional decolourization techniques, and the work by Banat and co-workers [10] offers a good review. Physical and chemical processes, often combined with biological treatments dominate the scenery, mainly flocculation, flotation, membrane filtration, ion-exchange, coagulation precipitation, ozonisation and activated carbon adsorption [11]. Some of them showed almost satisfactory performances for the removal of dyes from textile wastewaters, while the process for the removal of sulfonated azo dyes had some lacks in the effective colour reduction. Anyway, the efficiency of the treatments depended on the initial characteristics of the wastewaters; moreover, one of the main drawbacks is the production of chemical sludge that needs to be proper disposed and the high consumption of chemicals which negatively affects the treatment costs. Fung et al. [11] in their paper outlined how to choose the technique based on the characteristics of dyeing wastewater. For an example, ozone treatment could be used to improve the biodegradability only if major pollutants contain conjugated double bonds or aromatic groups [12]; coagulation could be used to remove colloids and suspended solids; other types of bio-treatments could be applied to degrade the wastewater according to the biodegradability of the suspensions [13]; ultrafiltration process are indicated to remove solids (1-20 nm) and dissolved organics and macromolecules with molecular weight between 300 and 300,000 g/mol.